This invention relates to a picture processing method in which a picture including half tone is restored (reproduced) with binary outputs.
In the case where picture signals read on the input side of a picture input and output device such as a digital type copying machine or facsimile equipment are transmitted to the output side thereof, a great problem is involved that is how to reproduce a picture substantially similar to an original picture with binary outputs on the output side while the picture signal transmission efficiency is improved in the decrease of the transmission capacity and in the reduction of the transmission time.
A method of reproducing a high tone picture by processing picture signals which are obtained by scanning or sampling an original picture such as a photograph including half tone by the picture element is known in the art. In this conventional method, a difference between the density level of an aimed picture element (i.e., a picture element to be processed) and the density level of the picture element which is positioned immediately before the aimed picture element is coded and transmitted to the output side, and in the output side the coded signal of the density level difference is decoded, so that a high tone picture is reproduced with multi-base or two-base (binary) outputs.
However, the conventional method is still disadvantageous in the following points: In order to reproduce a picture such as a photography including half tone with high quality in the conventional method, it is necessary to use high tone (at least sixteen tones) density levels. That is, the picture is sampled in at least sixteen tones. Accordingly, even if the aforementioned difference coding is carried out in the input side, an average bit length per picture element exceeds two (2) bits/picture-element (for instance, when a picture is sampled in 16 tones, the difference between the density level of an aimed picture element and that of the preceding picture element becomes frequently two (2), and if the difference is two (2), then the number of coded bits will be three), and accordingly the time required to transmit the coded signal to the output side is increased as much.
Accordingly, a first object of this invention is to provide a picture processing method, in which the transmission efficiency, especially the transmission time can be improved by sampling a picture in low tone, and yet a high tone picture excellent in picture quality can be restored (reproduced).
Furthermore, a so-called "systematic dither method" is also known as a method of reproducing a high tone picture with binary outputs, in which a picture such as photograph including half tone is scanned by the picture element, and the resultant picture signals are processed to provide the binary outputs.
In the systematic dither method, a picture made of picture elements arranged in matrix form is covered, in its entirety, by a sub-matrix of n.times.n arrangement, and a predetermined dither pattern is provided for the submatrix, so that the level of each picture element is determined as black or white with the dither values thereof as thresholds.
However, the conventional dither method is also disadvantageous in the following points: When a picture including half tone is sampled with (n picture elements/mm.times.m picture elements/mm), then the output of the picture is also of the same (n picture elements/mm.times.m picture elements/mm). Therefore, the resultant reproduced picture is low in resolution. For instance, in the case where a picture including half tone is sampled with (8 picture elements/mm.times.8 picture elements/mm) and the read picture data are subjected to dither process, the output picture is expressed by the black and white picture elements of (8 picture elements/mm.times.8 picture elements/mm). If, in this case, a dither matrix of 2.times.2 arrangement is used in the dither process, the resolution is not very lowered, but a high tone picture cannot be restored. If a dither matrix of 4.times.4 arrangement is employed, the reproduced picture may be of high tone, but the resolution is lowered; that is, the restored picture is low in picture quality.
Accordingly, a second object of the invention is to provide a picture processing method in which, in restoring a picture including half tone with binary outputs, a high tone picture excellent in resolution can be provided.
Furthermore, in the above-described systematic dither method, the picture signals which are provided by scanning the picture by the picture element with an image sensor are subjected to n-base quantization by an analog-to-digital converter, and the above-described process is carried out in accordance with the density levels of the thus-quantized picture signals, so that the picture is restored with the binary outputs. However, in this case, the number of tones of the restored picture cannot exceed n. For instance, where the picture signal quantization level in the input side is of 4-base, the number of tones of a picture restored in the output side is four or less. In contrast, in the case where it is required to provide a restored 8-tone picture in the output side, 8-base quantization must be carried out in the input side. Therefore, in the case of a 4-tone picture, a 2-bit analog-to-digital converter can be satisfactorily employed. However, in the case of an 8-tone picture, a 3-bit analog-to-digital converter must be employed, and in the case of a 16-tone picture, a 4-bit analog-to-digital converter must be used. Thus, as the number of tones in a picture is increased, the number of bits of an analog-to-digital converter used must be increased.
Furthermore, as the number of quantization levels in the input side is increased, the number of bits in transmitting the quantized signals to the output side is increased. Thus, the transmission efficiency is decreased, and the transmission time is increased.
Accordingly, a third object of this invention is to provide a picture processing method, in which a picture including half tone is quantized in low tone, thereby to reduce the number of bits in the quantized signals, and yet a high tone picture excellent in resolution can be restored with binary outputs.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.